Despite the key importance of plasma fibrinogen in hemostasis and thrombosis, very little is known about the synthesis of fibrinogen and its regulation. In this study we propose to examine the rate of synthesis of fibrinogen by rat hepatocytes using a newly developed monolayer cell culture technique. This technique is novel in that, in addition to its non-dependence on the addition of serum or other proteins in the culture media, the cells are viable for several days. The fibrinogen produced will be measured by a rat fibrinogen radioimmunoassay. The effects of drugs, hormones and physiochemical stimuli on the fibrinogen synthesis rate will be examined. The hypothesis that the plasma of patients with high fibrinogen levels contain a fibrinogen synthesis stimulating factor will be tested.